The diagnosis and treatment of a wide variety of medical conditions often involve the introduction of one or more agents into the body of a patient. The effectiveness of procedures that use such agents can often depend on the accuracy of the delivery of the agent to specific anatomical structures of the patient. In diagnostic contexts, for example, signal quality and diagnostic accuracy may be improved by the targeted administration of agents such as contrast media to the structures of interest. In treatment settings, the therapeutic effect of a specific pharmaceutical may be enhanced by the delivery of the pharmaceutical to the particularly affected structures of the body. Such enhancements may also have the added benefit of decreasing the risk of toxicity as lower dosages may be employed without substantial loss of therapeutic benefit.
One approach to the targeted administration of diagnostic or therapeutic agents involves combining the agent with a suitable carrier medium designed to deliver the agent to specific areas of the body of a patient. The specific carrier medium that is suitable in each application is selected for its particular combination of properties which account for the ability of the carrier medium to transport the agent to the structures of interest and to extend the residence time of the agent at such structures. Through the use of a carrier medium, the agent may be selected without regard to its own intrinsic abilities to travel to and remain at specific anatomical structures. As a result of the benefits of targeted administration, the use of a specifically designed carrier medium permits the effective use of a broad array of diagnostic and therapeutic procedures in which agents are used.
The ability of a carrier medium to transport an agent to and remain at a particular anatomical structure of a patient results from the physical and/or chemical properties of the carrier medium. In many applications, a suitable carrier medium is in the form of a two-phase system, such as a colloidal dispersion, comprising a continuous first phase into which a particulate second phase of a controlled size range is dispersed. In such applications, the properties of the colloidal dispersion as a carrier medium are affected by the particle size of the dispersed phase, the ability of the dispersed particles to combine with an agent, and its overall biocompatibility. In view of the specific anatomical structures through which the carrier medium transits and to which it is targeted, it is often desirable to use a dispersed phase having a narrow particle size range. By careful selection of particle size range, the carrier medium may be designed specifically for effective transit through particular vessels and persistent residence at particular anatomical structures.
By way of example, nanometer-sized carriers of a controlled size range are important for size-dependent diagnostic imaging such as imaging of sentinel lymph nodes in connection with the surgical treatment of various forms of cancers such as melanoma and carcinoma of the breast. A significant percentage of patients that present with such cancers show no clinical evidence of metastases while nonetheless harboring occult lymph node metastases. The presence of regional lymph node metastases is a very important predictor of patient survival, and prophylactic elective lymph node dissection has been shown to enhance survival rates when metastases are found.
Lymphoscintigraphy is an imaging technique based on the hypothesis that a melanoma or a breast carcinoma metastasizes to regional lymph nodes via a defined connection of lymphatics. Sentinel lymph nodes are the first lymph nodes in the regional basin which receive direct lymphatic drainage from the primary tumor and which can signal the spread of malignant cells. The absence of cancerous cells in the sentinel lymph nodes is considered to be strongly indicative of the absence of metastases to other nodes in the regional basin. Lymphoscintigraphy is used to image the drainage patterns of the lymphatic system so that sentinel lymph nodes may by located with accuracy. Specific drainage patterns are mapped by the administration of a radioactive colloid at or near the site of the primary tumor. As the colloid transits through the lymphatic system the drainage patterns from the primary lesion are defined and the sentinel lymph nodes may be located. If the sentinel lymph nodes are found negative for metastases, patients may be spared extensive node dissection surgery. If, however, sentinel lymph nodes demonstrate the presence of cancer cells, the patient may be properly identified for complete lymph node dissection and further study. The techniques and details of lymphoscintigraphy as disclosed in U.S. Pat. No. 5,732,704 to Thurston et al. and U.S. Pat. No. 6,205,352 to Carroll are incorporated herein by reference in their entirety.
The need for small and uniform colloidal particles used in lymphoscintigraphy arises from the particular characteristics of the lymphatic system. Particles smaller than about 4 to 5 nm tend to penetrate capillary membranes while large particles, above about 500 nm, become trapped in interstitial spaces or otherwise become stagnant. In either case, such particles do not migrate well through lymphatic channels. For these reasons, it is desirable that particles for sentinel lymph node imaging are uniformly small ranging in size from about 10 to about 200 nm. At present, colloids that have been used in lymphoscintigraphy include 99mTc-sulfur, 99mTc-albumin, 99mTc dextran, 99mTc hydroxyethyl starch, 99mTc human serum albumin, 99mTc-antimony trisulfide, and 99mTc-rhenium sulfide. These colloids, however, are not optimal as they lack a narrowly controlled size range and/or require dispersion in an organic continuous phase.
Other related technologies are described in the following publications each of which is incorporated herein by reference in its entirety: W. Stober, A. Fink, and E. Bohn, “Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range,” J. Colloid Interface Sci., 26, 62–69 (1968); E. P. Plueddemann, “Silane Coupling Agents,” Plenum Press, New York, Chapter 3, 49–73 (1982); A. van Blaaderen, and A. Vrij, “Synthesis and Characterization of Colloidal Dispersions of Fluorescent Monodisperse Silica Spheres,” Langmuir, 8 [12], 2921–2931 (1992); K. C. Vrancken, K. Possemiers, P. Van Der Voort, and E. F. Vansant, “Surface Modification of Silica Gel With Aminooorganosilanes,” Colloids and Surfaces, 98, 235–241 (1995); A. E. Hawley, L. Illum, and S. S. Davis, “Lymph Node Localisation of Biodegradable Nanospheres Surface Modified With Poloxamer and Poloxamine Block Co-polymer,” FEBS Letters, 400, 319–323 (1997); M. R. S. Keshtgar and P. J. Ell, “Sentinel Lymph Node Detections and Imaging,” Eur. J. Nucl. Med., 26[1], 57–67 (1999); A. J. Whilhelm, G. S. Mijnhout, and E. J. F. Farnssen, “Radiopharmaceuticals in Sentinel Lymph-Node Detection—an Overview,” 26, S36–S42I (1999); and C. Tsopelas, “Particle Size Analysis of (99m)Tc-Labeled and Unlabeled Antimony Trisulfide and Rhenium Sulfide Colloids Intended for Lymphoscintigraphic Application,” J. Nucl. Med., 42[3], 446–451 (2001).
It is known that colloidal silica can possess a narrow size distribution which can be altered easily by varying the parameters of the preparation method. Colloidal silica with a uniform and controlled size is commonly prepared using a sol-gel method. In this process, size-controlled silica particles are obtained through base-catalyzed hydrolysis and condensation. Alkoxysilanes such as tetraethylorthosilicate (TEOS) are hydrolyzed in an alkaline solution including ethanol and water, and silica particles are nucleated homogeneously. The silica particles then grow via consecutive hydrolysis and condensation. The size of silica particles can be controlled by modifications to various reaction conditions such as initial reagent concentration, reaction time, temperature and solvent.
In addition to controlled particle size, carrier media suitable for use in targeting specific anatomical structures must also be able to combine in a stable fashion with the selected diagnostic or therapeutic agent so that effective transport to the structures of interest is realized. One approach has employed the use of silica particles which have been modified with a silane coupling agent such as aminosilane. The amine groups which are bound to the surface of the silica particles enhance the carrying capacity of the particles by providing attachment sites at which the diagnostic or therapeutic agent may be fastened. The amine group is an especially versatile group in providing attachment sites for a wide variety of agents such as imaging materials for use in diagnostic procedures as well as pharmaceuticals and genetic material for use in therapeutic applications.
In diagnostic applications, colloids with agents immobilized on the surface of the particles comprising the dispersed phase are considered “labeled,” with the label being the immobilized agent. For example, in applications involving imaging techniques which use various frequencies of light, the label may be a fluorescent agent, for example, a fluorescently-labeled silica colloid. These colloids are prepared in an anhydrous solvent in order to prevent the hydrolysis and condensation of aminopropyltriethoxysilane (APTS), a commonly used silane coupling agent. Such colloids, however, are not suitable for biomedical applications in which an aqueous environment is preferred at near neutral pH.